In charged-particle-beam (CPB) projection microlithography as used in the fabrication of integrated circuits, a circuit pattern defined by a reticle or mask is irradiated with a charged particle beam (e.g., an electron beam) to transfer the pattern defined by the reticle or mask to a sensitized substrate (e.g., a semiconductor wafer). With certain conventional CPB projection-microlithography apparatus ("pattern-transfer apparatus"), one or more entire die patterns defined on a mask are transferred onto the wafer in a single exposure. Such a scheme is termed "batch exposure," and a "die" is a pattern coextensive with the bounds of an integrated circuit or other device to be transferred onto the wafer (usually multiple dies are exposed at respective locations on the wafer).
Unfortunately, although conventional batch pattern-transfer apparatus achieve good throughtput (i.e., the number of semiconductor wafers that can be exposed with a pattern per unit time), such apparatus cannot provide the high resolution and integration densities necessary to produce the semiconductor integrated circuits demanded in recent years. Specifically, it is difficult to produce a mask for batch transfer. In addition, conventional batch pattern-transfer apparatus have projection-optical systems in which the optical field is very large. Such projection-optical systems cannot satisfactorily control aberrations arising in the projection-optical system especially over such a large optical field.
To solve this problem, CPB pattern-transfer apparatus have been proposed in which the pattern to be transferred is divided into multiple field segments that are individually and separately exposed. Each field segment is typically further divided into multiple "mask subfields." Each mask subfield is typically rectangular with an area of hundreds of square micrometers. The mask subfields are typically transferred using a "step-and-repeat" transfer scheme in which the individual mask subfields are sequentially exposed onto corresponding "substrate subfields" on a wafer or other sensitized substrate. The substrate subfields are produced on the wafer surface in locations relative to each other such that the substrate subfields are "stitched" together in the correct order and alignment to reproduce the entire die pattern on the wafer surface (e.g., see U.S. Pat. No. 5,260,151). Such a transfer scheme is typically referred to as "divided exposure" mask-pattern transfer.
Conventional CPB pattern-transfer apparatus ("divided-projection apparatus") employing a mask segmented into subfields that are individually projected onto the water utilize projection-optical systems having a much smaller optical field than batch systems. As a result, compared to the batch systems, exposure of the mask pattern can be performed with better control of aberration such as field curvature, astigmatic blur and distortion.
However, in conventional CPB pattern-transfer apparatus, e.g., variable-beam-shaping (VBS) apparatus and cell-projection apparatus, a phenomenon known as a Coulomb effect can cause the focal-point position of a projected image to shift downstream along the optical axis. Focal-point shifting due to the Coulomb effect is dependent upon, inter alia, the total amount of current transmitted through the shaping aperture or cell patterns. Certain conventional CPB pattern-transfer apparatus (e.g., VBS or cell-projection apparatus) may correct axial shifting of the focal point. Such correction is usually performed using a "refocusing lens" that adjusts the focal-point position based on the transmitted beam current. A refocusing lens may also be used for divided-exposure transfer schemes to correct axial shifting of the focal point.
The Coulomb effect arises by mutual repulsion of the charged particles in the charged particle beam. In addition to causing axial shifting of the focal-point position, the Coulomb effect typically causes distortions and blurs of projected images. For VBS and cell-projection systems, distortions and blurs induced by Coulomb effects are not a problem due to the relatively small optical fields (e.g., about 5 .mu.m.times.5 .mu.m) of such systems. However, if the size of the optical field to be projected were to be substantially increased to, e.g., hundreds of square micrometers, then projected image distortions and blur induced by the Coulomb effect would be correspondingly increased. The lowest-order distortions caused by the Coulomb effect are linear distortions, which include magnification error, rotation error, and astigmatic-distortion error. For example, the lowest-order distortion corresponds to an image magnification change of 1/10000, and/or an image rotation of 10 .mu.rad. For conventional exposure systems such as VBS or cell-projection apparatus wherein the subfields are generally about (5 .mu.m.sup.2), the magnification error is typically less than 1 nm. However, with divided-exposure pattern-transfer systems, the subfields are typically several hundred square micrometers. Such distortions are then in the order of tens of nanometers, which distortions cannot be ignored. Thus, projected-image distortions of such magnitude must be corrected to provide integrated circuits having the high resolution and integration densities demanded in recent years.
Accordingly, there is a need for CPB pattern-transfer apparatus and methods for transferring a pattern from a divided-mask pattern onto a sensitized substrate with precise correction of focal-point shifting, low-order distortions, and blur of transferred images caused by the Coulomb effect.